This invention relates to an energization circuit, for a three-phase submersible pump motor, which reduces the motor voltage during the starting cycle.
Reduced voltage starting of three-phase submersible pump motors is especially desirable for shallow well installations (such as those wells less than 2,000 feet deep) where the relatively short cables feeding the motors introduce negligible voltage drops or line losses. With substantially full voltage from the surface a-c power source available at the motor terminals at start-up, very high starting current (and consequently high starting torque) will result and this can cause motor shaft failure from mechanical fatigue. The high starting current occurs since the motor, upon starting, appears as a low impedance to the power system. In contrast, in normal applications (namely, deeper well installations), the cable supplying power to the submersible pump motor provides a large voltage drop (a high line loss) which in turn decreases the starting torque and therefore the torque stresses on the shaft. In effect, the long cable for a deep well installation causes a voltage sag during start-up.
Several low voltage starting arrangements for motors have been developed in the past. For example, autotransformers with tap changers have been used in motor energizing circuits to produce low voltage on starting, the tap being switched after the starting cycle is completed. In other cases, inductors have been switched in series with the motor at start-up to introduce a voltage drop. Other examples of prior low voltage starters involve resistors and capacitors that are inserted into the energizing circuit.
The present invention provides a unique low voltage starting arrangement and constitutes a significant improvement over those previously developed. Morover, the present scheme is more efficient and cost effective than the prior approaches. Furthermore, no circuit components need be switched in and out of the motor energizing circuit (namely, no moving parts) in switching between starting and running conditions.